The qualitative and quantitative determination of cations is of major significance in areas such as chemical and biochemical engineering for process control, in agriculture chemistry for soil research and fertilizer metering and in medicine for diagnostic and therapeutic determination of the potassium-sodium ratio. Present methods for cation determination include flame photometry and atomic absorption spectroscopy, both of which require sophisticated apparatus. Ion-sensitive cation electrodes on an ion-exchange basis generally yield sufficiently differentiated results, but are cumbersome to use.
Vogtle, U.S. Pat. No. 4,367,072 describes a process for determining ions. It is essentially based on selective complexing between the ion to be determined and a complexing agent and measurement of the extinction change occurring during complexation. The complexing agent is bonded with a chromophore.
The selective complexing agent may be an oligoether, oligoester or oligoamide, containing, for example, corand, cryptand or cyclic peptide and/or polyethylene glycol groups or other hetero atom-containing groups. The covalently or heteropolarly bound chromophore is a dye or fluorescent dye or a chromogen whose absorption spectra change through interaction with ions or lipophilic molecules through charge shifts or disturbances of the mesomeric system. This principle is well known in nature and in the art. Hemin, chlorophyll and metal complex dyes and metal indicators (e.g., zylenol orange and methylthymol blue based on the colorless complexing agent ethylenediaminetetraacetic acid (EDTA)) exhibit, to a greater or lesser extent, this general configuration.
A general problem of the above-cited complexing agents is that they usually are capable of reacting only in organic media, whereas the ion being determined is, as a rule, present in an aqueous solution. Although the aqueous solutions of the ions could be transformed in many cases to organic media by concentration, inclusion in organic salts, or solvent extraction, this would not satisfy the requirements of a practical and, if necessary, automated rapid method.
Klink, et al., European Patent Publication 85,320, disclose a potassium reagent and a procedure for determining potassium ions. The reagent contains a compound of general formula ##STR5## where n and m=0 or 1, X=N or COH and R=p-nitrophenylazo, 3-phenylisothiazolyl-5-azo, isothiazolyl-5-azo, thiazolyl-5-azo, 2,4,6-trinitrophenylazo, p-nitrostyryl, p-benzoquinonemonoimino and bis-(p-dimethylaminophenyl) hydroxymethyl. The potassium ions are determined in a reaction medium consisting of water and at least one water-miscible organic solvent and in the presence of an organic base.
Klink et al. do not recognize the interference from sodium ion in determination of potassium in EP 85,320. They provide extinction maxima data of various cations, and state that aside from the extinction maxima for rubidium, all other extinction maxima for the various cations tested are so far removed from potassium's extinction maxima that no interference occurs. However, Klink et al. base their conclusion on data obtained from isolated cation measurements, and fail to contemplate the effect on extinction maxima for these cations in mixed cation solutions.
The present invention is directed to novel compounds, reagents and methods which permit rapid determination of the presence of cations in a sample. The present invention also concerns reagents and methods permitting rapid determination of cations in single-phase aqueous media, wherein one of the improvements comprises use of one or more interfering cation complexing compound masks.
Certain cryptands have high selectivity for complexing with cations and, if coupled with chromophores, yield intensive color reactions that can be evaluated analytically. For example, it has been discovered that chromogenic cryptand 3.2.2 is particularly effective for potassium cation determination. Furthermore, it has been discovered that chromogenic cryptand 3.3.2 has good sodium cation selectivity. Determination of cations is further enhanced by using reagents of the present invention which may also contain one or more interfering cation complexing compound masks. For example, reagents and methods of the invention are effective for determining potassium ion concentration of a sample comprising a mixture of potassium and sodium ions. Reagents and methods of the invention are also effective for determining sodium ion concentration of a sample which comprises a mixture of large amount of sodium ions and potassium ions. These and other advantages will be more clearly described in the detailed description of the application.